1. Field of the Invention
This invention relates to copolymers of C.sub.1 -C.sub.5 alkyl vinyl ether and maleic acid, and, more particularly, to colorless, stabilized aqueous solutions of such acid copolymers, preferably by hydrolyzing the corresponding anhydride copolymer before drying it to a powder.
2. Description of the Prior Art
The copolymerization of maleic anhydride and a C.sub.1 -C.sub.5 alkyl vinyl ether yields a copolymer which is more properly characterized as an interpolymer wherein both ethylenically unsaturated moieties enter into the formation of the same polymer chain. It has been indicated by infra-red spectroscopy, nuclear magnetic resonance spectroscopy and other chemical analyses that alkyl vinyl ethers and maleic anhydride interpolymerize in the molar ratio of about 1:1, the polymer chain thereof containing alternating anhydride units and alkyl vinyl ether units. The resulting anhydride interpolymers can be hydrolyzed in water to the corresponding acid interpolymer. Such acid interpolymers are known to be useful in personal care products and other applications.
Generally, copolymers of maleic acid and an alkyl vinyl ether, e.g. methyl vinyl ether, e.g. Gantrez.RTM. 97, are prepared by hydrolyzing powders of the anhydride precursor copolymer, e.g. Gantrez.RTM. AN-169.
Under normal ambient storage conditions, however, it is known that copolymers of maleic acid and an alkyl vinyl ether prepared from the anhydride powder will degrade with time, as evidenced by a decrease in molecular weight and viscosity of the copolymer.
Kervinski, in U.S. Pat. No. 3,531,427, found that the molecular weight of such copolymers could not be stabilized with only a single stabilizing agent, but that it could be stabilized by using a mixture consisting of selected oxidizing and/or reducing agents, e.g. ethylenediamine tetraacetic acid (EDTA), in combination with other secondary additives. The use of such a stabilization mixture, however, may not be suitable, due to the toxicity of the secondary agent, or discoloring of the copolymer solution in the presence of the stabilizer mixture.
Frosch, et al, in Ger. Patent 4,213,972 A1, described the use of a single stabilizer agent for copolymers of maleic acid or maleic anhydride and alkyl vinyl ethers which was a phenol containing a tert-butyl group. However, in general, phenols are not favored due to their toxicity.
Accordingly, it is an object of this invention to provide a colorless, stabilized aqueous solution of a copolymer of a C.sub.1 -C.sub.5 alkyl vinyl ether and maleic acid which is suitable for personal care and industrial applications, using a single stabilizing agent which is effective for stabilizing the copolymer even when used at low concentrations.
Another object of the invention is to provide a process of preparing such colorless, stabilized aqueous solutions having higher viscosities than available previously and which will retain their initial viscosity levels over extended periods of time.
A feature of the invention is a process for stabilizing such copolymers in which the acid copolymer is made by hydrolyzing the corresponding anhydride copolymer while it is still in a wet state, i.e. before drying it to a powder.